Passive mode locking of lasers has been achieved by placing a saturable absorber within the lasing cavity. Passive mode locking of a semiconductor laser was described by van der Ziel et al. in an article entitled "Subpicosecond Pulses from Passively Mode-Locked GaAs Buried Optical Guide Semiconductor Lasers", in the journal Applied Physics Letters, Vol. 39, No. 7, pp. 525-527, October 1981. Van der Ziel et al. used proton bombardment near one end of the active region of a buried optical guide semiconductor laser in order to make absorption centers which served as a saturable absorber. A difficulty with this approach is that absorption center density and placement are hard to control and not accurately reproducible when proton bombardment is used to make the absorption centers.
Passive mode locking of semiconductor lasers has proven difficult because of the high intensities needed to saturate the usual nonlinear absorbers used to mode lock lasers. The materials used as absorbers to mode lock dye lasers or gas lasers require intensities which are generally out of the range of semiconductor lasers.
Another approach to control of semiconductor laser gain is to divide the semiconductor into two regions and to separately control the currents in each region. One region is biased to lase, and the other region is biased below threshold so that it behaves as an absorber wherein the absorption is controlled by control of the current. Active mode locking of a semiconductor laser is achieved by driving the control current at a frequency equal to the frequency difference between modes. However, only active mode locking and not passive mode locking is achieved by this method.
Also, control of the gain of a semiconductor laser may be achieved by simply controlling the pumping current. By increasing the current above threshold, the laser is turned on, and by decreasing the current below threshold, the laser is turned off. A difficulty with controlling the pumping current is that abruptly turning the laser on and off introduces unwanted spectral components in the spectrum of the output light. Highspeed modulation of the intensity of the output light is not satisfactorily achieved by this method, especially when a narrow frequency band is desirable in the spectrum of the output light.
No satisfactory method for directly controlling the gain of a laser by an external light source has been advanced. Such control is important in integrated optical circuits in which all optical logic is performed.